This invention relates generally to data transmission systems and specifically to a data transmission system for transmitting MPEG (Motion Picture Experts Group) data along an ATM (Asynchronous Transfer Mode) network such as those used by telephone companies. The MPEG information is provided by one or more servers that deliver complete MPEG 2 type programs of constant data rate, that is comprising a fixed number of megabits per second. Although fixed for any given program, the data rates may differ depending upon the program. The ATM network, on the other hand, transmits at a constant data rate and utilizes data packets that differ from the MPEG data packets. Conversion from the MPEG format to the ATM cells and multiplexing of the ATM cell information from the plurality of servers are involved. The system chosen for purposes of description delivers MPEG 2 data to 16 VSB (vestigial sideband) modulators in a cable plant. The servers may be situated at different locations with their bit rates ranging from 0 to 15 megabits per second. The bit rate for each server is constant (for a given program). The disclosed system shows a 155 megabits per second constant ATM bit rate feeding four 16 VSB format modulators. One hundred and twenty eight servers are indicated although this number is arbitrary.
Data sent through an ATM system will incur a non-constant delay. Thus, for example, a series of ATM cells spaced 100 microseconds apart when they enter the ATM network, will be spaced 50, 67, 2, 200 etc. microseconds apart when they exit the ATM network. This variation is called "jitter".
As mentioned, the ATM system formats the MPEG 2 data into ATM cells via a series of N MPEG to ATM converters (MTA) and multiplexes the ATM cells during transmission. Each pair of MPEG 2 packet is converted into eight ATM cells which form an ATM packet. The ATM network guarantees that the cells from each server will remain in their original order although the cells will be intermixed with cells from other servers. The data from the ATM network is received by an ATM interface module (AIM) and supplied to an MPEG processing module (MPM) which reconverts the ATM protocol to MPEG protocol. The conversion is made by writing the ATM cells to one of N queues with an individual queue being provided for each server or program. The queues output the two original MPEG data packets that make up each ATM packet. These output MPEG packets are multiplexed under the control of a microprocessor and applied to an MPEG packet router which delivers the MPEG packets to one of four buffers that in turn supply the information to corresponding ones of four 16 VSB modulators. Each modulator (which corresponds to a cable transmission plant or site) supplies a number of set top boxes (STBs) which are built into or associated with individual receivers such as television receivers. Each set top box includes a 16 VSB demodulator, an MPEG transport chip, a rate buffer and an MPEG video/audio decoder. The decoder feeds a receiver.
A difficulty arises due to the problem of jitter and in particular the problem that jitter has on programs requiring a higher bit rate. The problem is concerned with the FIFO format of the queues. It is presently contemplated that as the final cell of an ATM packet (two MPEG data packets) is received in a queue, that pair of MPEG packets is ready to be multiplexed and sent to the packet router. After the pair of MPEG packets is sent out, the next completed group of two MPEG data packets is sent out, and so forth. The arrangement magnifies the jitter. As will be shown, a relatively small amount of jitter in the ATM network may generate a significantly larger amount of jitter when the ATM packet data is reconverted to MPEG data form and sent out to the individual STBs in the system. In particular the individual rate buffers (at the receiver locations), which are required to supply a constant bit rate of information to the decoders, need to be large to accommodate the maximum jitter or delay expected in the MPEG data supplied thereto.
The present invention minimizes the jitter magnification problem with the result that the rate buffers for the individual STBs can be smaller.